For a while, Travis CI was a go-to staple for CI on a lot of GitHub projects. Free and reliable. Unfortunately, since they were acquired, they’ve changed a lot of their free availability, they’ve had an annoying number of outages, and their reputation is kind of dropping quickly.
Which leads me to not necessarily believe that Circle CI is the greatest option for the future. There’s nothing really wrong with it right now, but I just don’t see why it can’t be goofed like Travis was. I definitely wouldn’t call CircleCI “industry standard,” but would hand that title to Jenkins, for better or for worse.
Github Actions is, imo, probably just more reliable for the future given how tightly it integrates with GitHub, which won’t be going away anytime soon. GitLab also has a pretty good CI featureset available, although I don’t know anything about its pricing model.
GH Actions has a bunch of good examples here.
There’s also a growing community for GH Actions plugins, for example, automatic code coverage comments on PRs and many others, which makes a very strong case for GH Actions.